In general, various electronic apparatuses such as home electronics, industrial apparatuses, and vehicle-mounted apparatuses are configured by a plurality of circuits. For example, a power conversion apparatus represented by an inverter apparatus that variably controls the speed of a motor is configured by a main circuit including a power module that supplies alternating-current power to the motor, a peripheral apparatus circuit that drives a cooling fan to cool the heated power module and operates apparatuses not directly related to control and operation of the motor, a control circuit that is mounted with an arithmetic processing device such as a microcomputer, a CPU, an ASIC, or an FPGA and transmits a motor driving signal to the main circuit, which drives the motor, and controls the overall control of the power conversion apparatus such as the operation of the peripheral apparatus circuit, a power supply circuit that supplies electric power serving as a power source for these circuits, and the like.
In general, as the power supply circuit, a switching power supply circuit with high conversion efficiency is used. In the case of the power conversion apparatus explained above, to eliminate the influence of noise transmitted from a main power supply serving as the power source for the power supply circuit or for a reason such as electric shock prevention, a switching power supply circuit including an insulated transformer is used.
The switching power supply circuit is configured by components such as a switching element, an insulated transformer including a primary winding wire and one or a plurality of secondary winding wires, a main power supply being connected to the primary winding wire via the switching element, a diode connected to the secondary winding wire(s) of the insulated transformer, a capacitor connected to the secondary winding wire(s) of the insulated transformer via the diode, and an IC for control power supply control that controls ON/OFF of the switching element. The IC for power supply control controls ON/OFF of the switching element on the basis of a voltage value of the capacitor. Basically, the switching power supply circuit controls a flow of electric power according to this ON/OFF operation.
When the insulated transformer includes one secondary winding wire, the IC for power supply control performs the control of ON/OFF of the switching element on the basis of the voltage value of the capacitor connected to the secondary winding wire via the diode. On the other hand, when the insulted transformer includes a plurality of secondary winding wires, in general, the IC for power supply control performs the ON/OFF control of the switching element on the basis of the voltage value of the capacitor connected to a certain one secondary winding wire via the diode.
In general, the IC for power supply control has a function of performing the ON/OFF control of the switching element on the basis of the voltage of the capacitor connected to the secondary winding wire(s) via the diode and a function of monitoring an electric current flowing to the primary winding wire and the switching element and, when an excessively large electric current flows, performing overcurrent protection for forcibly turning off the switching element.
The switching power supply circuit has a state in which, when a power supply is turned on, there is no voltage of the capacitor connected to the secondary winding wire(s) of the insulated transformer via the diode (hereinafter referred to as “start time or during start”) and a state in which electric power generated by the secondary winding wire(s) of the insulated transformer is stably supplied and the voltage of the capacitor reaches a desired voltage set in advance (hereinafter referred to as “commanded voltage”) (hereinafter referred to as “steady time or steady state”).
At the start time, because power energy accumulated in the primary winding wire of the insulated transformer increases, an excessively large electric current flows to the primary winding wire and the switching element. Therefore, the overcurrent protection operation of the IC for power supply control works and the switching element is forcibly turned off. Thereafter, the overcurrent protection operation is released and the switching element is turned on again. However, the switching element is turned off by the overcurrent protection operation performed again. This operation is repeated, a voltage is charged in the capacitor connected to the secondary winding wire(s) via the diode, and the switching power supply circuit changes to the steady state.
On the other hand, in the steady state, the voltage of the capacitor connected to the secondary winding wire(s) via the diode is charged to the commanded voltage, an excessively large electric current does not flow to the primary winding wire and the switching element. However, when there is some abnormality in the switching power supply circuit, the circuit connected to the secondary winding wire(s), or the like and an excessively large electric current flows to the primary winding wire and the switching element, the overcurrent protection operation of the IC for power supply control works.
The overcurrent protection operation of the IC for power supply control is for preventing thermal destruction of the switching element and the diode, which are the components of the switching power supply circuit, magnetic saturation of the insulated transformer, and the like. If a current value for operating the overcurrent protection (hereinafter referred to as “overcurrent protection level current value IDOC”) can be reduced, when some abnormality occurs in the switching power supply circuit, the circuit connected to the secondary winding wire(s), or the like and an excessively large electric current flows to the primary winding wire or the switching element, the operation of the switching element can be quickly interrupted. Therefore, as the switching element and the diode having small heat capacities can be adopted, and the magnetic saturation of the insulated transformer can be prevented, a reduction in the size of the insulated transformer can also be attained.
For example, when a load of the circuit connected to the secondary winding wire(s) via the diode increases because of some abnormality, an excessively large electric current flows to the secondary winding wire(s) and the diode and, and at the same time, an electric current flowing to the primary winding wire and the switching element also increases. If the electric current flowing to the primary winding wire and the switching element reaches the overcurrent protection level current value IDOC, the switching element is forcibly turned off and the power supply to the secondary winding wire(s) is stopped.
However, when the overcurrent protection level current value IDOC is reduced, at the start time of the switching power supply circuit, sufficient power energy cannot be accumulated in the primary winding wire and a desired voltage cannot be charged in the capacitor connected to the secondary winding wire(s) via the diode. Therefore, it is necessary to set the overcurrent protection level current value IDOC with which the commanded voltage can be charged in the capacitor.
Because of the reasons explained above, to set the overcurrent protection level current value IDOC necessary at the start time, the switching element and the diode having large heat capacities unnecessary at the steady time are made necessary. At the same time, there is also a problem in that the insulated transformer is also increased in size for magnetic saturation prevention of the insulated transformer, the switching power supply circuit is increased in size, and costs increase.
As measures against the problems, Patent Literature 1 described below discloses a technology for changing the overcurrent protection level current value IDOC of the IC for power supply control, that is, a technology for monitoring, with the IC for power supply control, a voltage value of the capacitor serving as a power source of the IC for power supply control and connected to an auxiliary winding wire by the diode and changing an overcurrent protection level current for an electric current flowing to the primary winding wire and the switching element. For example, when the voltage value of the capacitor is low, it is determined that the switching power supply circuit is at the start time or an overload, a short circuit, or the like of the circuit connected to the secondary winding wire(s) has occurred, and the IC for power supply control operates to set a small overcurrent protection level current value IDOC (hereinafter referred to as “overload time overcurrent protection level current value IDOC”). When the voltage value of the capacitor reaches the commanded voltage, it is determined that the switching power supply circuit is in the steady state. The IC for power supply control operates to set the conventional overcurrent protection level current value IDOC (hereinafter referred to as “conventional overcurrent protection level power supply value IDOC”; and is set to a value larger than the overload time overcurrent protection level current value IDOC). Only at the start time, a change of the overcurrent protection level current value IDOC is prohibited to quickly perform the start of the switching power supply circuit. According to the operation explained above, the heat capacities of the diode and the switching element can be reduced. Therefore, it is possible to reduce the size of the components of the switching power supply circuit.